In order to form a joint between a corrosion resistant metal tube and a corrosion susceptible metal tube, a mechanical fitting has been employed. For instance, a titanium tube can be welded to a titanium portion of the fitting with an O-ring seal placed intermediate between the titanium and copper portions of the fitting which are, in turn, soldered to a copper tube. Titanium heat exchanger tubing for use with a swimming pool heat pump can withstand chlorine levels one-thousand times higher than that experienced in a swimming pool without corroding or pitting. This is a significant improvement over the prior art use of copper-nickel alloys for swimming pool heat exchangers. Such alloys can corrode rapidly if the chlorine level in the pool becomes too high, allowing water to mix with the FREON.RTM. in the refrigeration system which, in turn, will destroy the compressor and the evaporator, resulting in a complete loss of the heat pump unit.
With the advent of titanium heat exchangers for swimming pool heat pumps, a solution to the problem of metallurgically bonding copper to titanium was sought in order to provide a metallurgically bonded connection of the titanium and copper tubing in the system. Difficulty has been experienced in making such connections of the titanium tubing for the heat pump heat exchanger to the copper tubing leading to the heat pump compressor and evaporator. Since titanium cannot be welded, brazed or soldered to copper, mechanical joints such as have been described above have been used. Such mechanical joints ultimately fail because of vibration in the system or failure of the O-ring seal.
In U.S. Pat. No. 3,791,026 a method of forming a joint between tubular parts, one of which is made of niobium and the other of stainless steel is disclosed. The tubes are assembled cylindrically one inside the other with provision for a clearance, the steel tube being located externally and provided at its extremity with an external machined annular flange forming a reservoir at the joint which is filled with a suitable brazing compound. The joint is subsequently heated under vacuum at 1,020.degree. to 1,030.degree. C.
Titanium clad steel plate, particularly that manufactured by the explosion bonding process, has become quite popular for the fabrication of chemical process equipment for use under highly corrosive conditions. To fabricate equipment, it is necessary to weld seams or joints at various sections. This poses several problems because of the requirements for welding titanium. Welding of the steel segment of the titanium clad steel plate must be accomplished without contacting the steel with the titanium metal. Welding of the titanium segment must be accomplished without contacting the titanium with the steel. If such contact occurs, either the titanium metal cladding or the titanium weld is ruined. Such contact causes the weld to crack. Welding titanium also requires the use of one of the well-known inert gas welding methods, e.g., the heliarc welding process. In the past, welding of such clad steel plates has required removal of the titanium cladding from the area of the steel weld and, after the steel weld is complete, adding a titanium filler strip and then covering the entire area with a second, wider titanium strip which is then welded to the titanium clad. While seams can be welded by this procedure, it is time consuming, requires an extra titanium filler strip, and can cause problems should leaks, particularly pinhole leaks, occur. In any case, an easier, less costly and more easily repaired system for welding titanium clad steel is needed.
It is an object of the present invention to provide a method of welding titanium clad steel. It is also an object to provide a method for welding titanium clad steel without contacting the titanium weld with the underlying steel. It is also an object of the present invention to provide a method of welding titanium clad steel that does not require an extra titanium filler strip.
Among the characteristics of steel which contribute to its widespread use in pressure vessels are its strength and availability at reasonable cost, but steel is attacked by many chemicals and cannot be exposed to such chemicals, particularly at elevated temperatures and pressures. To offset this problem, it has been common practice to clad steel with less reactive, yet more expensive, value SAL 2/15/00 meter, such as tantalum, zirconium and titanium and their alloys. This, however, presents a whole new set of problems.
Some of the so-called value SAL 2/15/00 metals which are currently used as cladding for steel plate have relatively high melting points which are considerably higher than that of steel. A weld made along the cladding will melt the steel beneath the cladding even though the weld does not fully penetrate the cladding. When the steel again solidifies, its physical characteristics are changed, and the result is a region of weakness in the steel backing. This problem has been overcome by interposing a layer of copper between the refractory metal cladding and the steel backing. The copper acts as a heat sin, and although it melts when the refractory metal is welded, it distributes the heat over a widespread area, and this prevents the underlying steel from melting.
The typical pressure vessel possesses a generally cylindrical configuration and often has hemispherical heads. As such, it is fabricated from various components and segments which are welded together. For example, the hemispherical heads each constitute separate components as does the cylindrical side wall which is interposed between them. Often the heads and the side wall are themselves fabricated from a multitude of segments. These components and segments must be welded together in a manner which presents a totally inert surface toward the interior of the vessel.
U.S. Pat. No. 3,443,306 entitled Method of Joining Clad Material discloses one procedure for welding together clad steel components when the cladding is tantalum which is separated from the steel backing by a copper intermediate layer. More specifically, the tantalum cladding and copper intermediate layer are stripped away from the steel backing at each edge where the joint is to be formed and when the two components are brought together, this creates a groove in the otherwise continuous layer of cladding. The groove exposes the steel backing for each component in the region of the joint, and here the two components are welded together along a butt weld in the steel. Next a filler strip of copper is inserted into the groove and either tack welded or continuously welded in place, and then a flat batten strip, which is formed from the same metal as the cladding, is placed over the copper filler strip. The width of the batten strip exceeds the width of the groove so that the edges of the batten strip overlie the cladding on the two components, and along these edges the batten strips are welded to the cladding. Again, care must be exercised to prevent the weld from fully penetrating the cladding, for any total penetration will draw molten copper into the weld and render it brittle.
While the foregoing procedure seems relatively simple, it is not. First, the batten strip is difficult to center over the groove and filler strip because it completely obscures the groove. Should it not be centered, the prospects of melting the filler strip or the weld metal which secures it are increased, and of course whenever such a melt occurs, copper or some other foreign metal is drawn into the tantalum weld to weaken it. Secondly, it is common practice to tack weld the batten strip in place before the fill welds are made along its edges, but the batten strip, being straight in cross section, has a tendency to distort when tack welded. Finally, the copper filler strip and the welds which are used to secure it, represent additional material which increases the cost of the whole procedure.
In addition, the procedures for welding titanium clad steel in U.S. Pat. No. 3,733,686 and tantalum clad copper in U.S. Pat. No. 4,688,691 are also of interest.